This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Formula display:

Abstract

In this work, we report a direct synthesis of vertically aligned ZnO nanowires on
fluorine-doped tin oxide-coated substrates using the chemical vapor deposition (CVD)
method. ZnO nanowires with a length of more than 30 μm were synthesized, and dye-sensitized
solar cells (DSSCs) based on the as-grown nanowires were fabricated, which showed
improvement of the device performance compared to those fabricated using transferred
ZnO nanowires. Dependence of the cell performance on nanowire length and annealing
temperature was also examined. This synthesis method provided a straightforward, one-step
CVD process to grow relatively long ZnO nanowires and avoided subsequent nanowire
transfer process, which simplified DSSC fabrication and improved cell performance.

Keywords:

Background

Dye-sensitized solar cells (DSSCs) have attracted significant research interest due
to their promising power conversion efficiency and low fabrication cost [1]. Typical photoanodes of DSSCs are layers of nanoparticles of wide band gap semiconductors
such as TiO2 or ZnO, and the substrates are usually fluorine-doped tin oxide (FTO)-coated glasses
[2]. However, in these nanoparticle-DSSCs, photo-generated electrons have to percolate
through the nanoparticle network before they reach the collection electrode, which
increases charge recombination possibility and limits cell performance. One approach
to improving charge collection efficiency in DSSCs is to replace the nanoparticle
network with one-dimensional structures such as semiconductor nanowires that can provide
direct transport pathway for the carriers. Due to the enhanced diffusion length, longer
wires and thus thicker photoanode films can be incorporated into DSSCs, which could
lead to better quantum efficiency in the long-wavelength region of the solar spectrum
[3]. In addition, recent studies also show that the open-circuit voltage of DSSCs can
be improved by employing nanowire-photoanodes, which is attributed to a suppressed
back electron transfer reaction that occurs at the photoanode/redox electrolyte solution
interface, highlighting the importance of exploring nanowire-based photoanodes for
DSSC applications [4,5].

There has been a significant amount of reports on DSSCs based on nanowires, where
the semiconductor nanowires are mainly synthesized using solution-based hydrothermal
method [6-8]. Using this method, the nanowires can be directly grown on FTO-coated substrates,
which make subsequent solar cell fabrication straightforward. However, solution-based
synthesis is usually slow and involves multiple processes, and post-growth annealing
is necessary to remove the unwanted chemicals from the nanowire surface and ensure
good electrical contact between the wires and the substrate [9]. Furthermore, it is generally difficult to produce long wires using the hydrothermal
approach. Due to the multiple steps involved and the low growth rate, it is very time
consuming to synthesize nanowires with a length of more than 10 μm [10-12]. Another popular nanowire synthesis approach is the chemical vapor deposition (CVD)
method that is based on vapor–liquid–solid (VLS) growth mechanism. Nanowires with
very long length can be synthesized this way; however, the substrates used are typically
silicon or sapphire other than FTO-coated glasses since the high CVD growth temperature
can easily damage the transparent conducting oxide [13-15]. In addition, since the nanowires are not directly synthesized on FTO-coated substrates,
a nanowire transfer process is needed in the subsequent solar cell fabrication. Such
a transfer process causes contact issues between the nanowires and the substrate as
well as broken wires in the device structure that creates additional transport barriers
and recombination possibilities for photo-generated electrons, which all could limit
solar cell performance.

Thus far, there is only limited research on direct synthesis of nanowires on FTO-coated
substrates using the CVD method, and the reported nanowires were not aimed at DSSC
applications and had low density and random morphology [16,17]. In this work, we investigated a controlled CVD synthesis of nanowires directly on
FTO-coated glass substrates. Long, vertically aligned ZnO nanowires were fabricated
at a relatively low temperature of 550 °C, and they formed dense arrays with length
of tens of microns in a one-step vapor deposition process. DSSCs were fabricated using
these directly grown nanowires, and the performance was compared to those fabricated
using transferred ZnO nanowires. The effects of nanowire length and annealing temperature
on device performance were also examined.

Methods

Direct growth of ZnO nanowires on FTO substrates by the CVD method

Vertically aligned single crystalline ZnO nanowires were synthesized directly on FTO-coated
glass substrates in a horizontal tube furnace at a low temperature by the chemical
vapor deposition method, where the nanowire growth followed a self-catalytic vapor–liquid–solid
mechanism [16,18,19]. Figure 1 shows the schematic of the system setup. A 1-in. quartz tube was mounted on a single-zone
furnace with a constant temperature heating zone of about 13 cm long. As shown in
Figure 1, a specially designed cylindrical sapphire source container with an inner diameter
of 1.4 cm, outer diameter of 2.0 cm, and length of 2.5 cm was used in this experiment.
The container with 0.3 g zinc powder (100 mesh, 99.9%, Alfa Aesar, Ward Hill, MA,
USA) as the source material was placed at the center of the tube. FTO-coated glass
substrate (TEC15, MTI, Richmond, CA, USA) with a size of 1.0 × 1.5 cm was first cleaned
by acetone and isopropyl alcohol and then covered by a Si3N4 shadow mask with a 0.5 × 0.5 cm square opening at the center. The substrate, together
with the shadow mask, was placed inside the tube at a distance of 0.5 cm downstream
from the source container. The tube furnace was first pumped down to a base pressure
of 10−2 Torr using a rotary pump, and then, it was heated up to 550 °C under a ramp rate
of 50 °C/min and a carrying gas mixture of N2 (100 sccm) and O2 (4 sccm). The temperature was maintained at 550 °C, while the pressure was kept at
8 Torr to allow the nanowires to grow.

Figure 1.Schematic of the system setup for nanowire synthesis. (not drawn to scale).

The as-grown ZnO nanowires were ready for solar cell fabrication without any further
processing. To sensitize the nanowires, the FTO substrate with the ZnO nanowires was
soaked in a 0.05-mM solution of N719 dye (dissolved in dry ethanol; SOLARONIX, Aubonne,
Switzerland) at 50 °C for 2 h. Another FTO substrate coated with 25 nm Pt was used
as the counter electrode and was bonded together with the nanowire/FTO substrate through
a hot-melt spacer (75 μm; Bynel, Dupont, Wilmington, DE, USA). A drop of electrolyte
(0.5 M LiI (Aldrich, St. Louis, MO, USA), 50 mM I2 (Alfa Aesar), and 0.5 M 4-tertbutylpyridine (Aldrich) in 3-methoxypropionitrile (Aldrich))
was injected into the space between the two electrodes of the cell. Current density-voltage
(J-V) curves were acquired by a source measurement unit (Agilent 4156 Semiconductor Parameter
Analyzer, Agilent Technologies, Santa Clara, CA, USA) under a simulated sunlight (100 mW/cm2, calibrated by a KG-5 filtered silicon photodiode) using a setup with a Xenon lamp.
The optical absorption of the dye solution was characterized by an ultraviolet–visible
(UV–vis) spectrophotometer (Lamda 950, PerkinElmer, Waltham, MA, USA).

DSSCs based on transferred ZnO nanowires were also fabricated and tested in this research
for the purpose of a comparison study. Since it was very difficult to remove the directly
synthesized nanowires from the FTO substrates, the transferred ZnO nanowires were
those grown on silicon substrates. The solar cell fabrication procedure was almost
identical, except that a nanowire transfer process was involved. To transfer the ZnO
nanowires, a polydimethylsiloxane (PDMS) solution was first spin-coated on the silicon
substrate with the ZnO nanowires, which, after annealing, would form a flexible but
solid film that holds the nanowires in position [20,21]. After being annealed at 150 °C on a hot plate in the air, the nanowire film was
peeled off by a sharp razor blade. The nanowire film was then soaked in the N719 dye
solution for 2 h at 50 °C, which was the same sensitization condition for the DSSCs
based on directly grown ZnO nanowires. After dye sensitization, the film was transferred
onto an FTO substrate and was glued down using a thin layer of silver paste. Since
the silver paste was easy to dissolve in a dye solution, dye sensitization was performed
before the nanowire film attachment, which was different from the previously reported
procedure [20].

Results and discussion

Figure 2a shows the field emission scanning electron microscope (FESEM) image (tilted at 15°)
of the as-grown ZnO nanowires on an FTO substrate, and the inset is a higher-magnification
image. The needle-shaped nanowires were vertically aligned, with a hexagonal face
on the tip of each nanowire. The X-ray diffraction (XRD) pattern in Figure 2b reveals the single crystalline structure of the wires with a [0001] growth direction,
which is consistent with the transmission electron microscopy examination of a single
nanowire that is shown in the inset of Figure 2b. The top and bottom diameters of the needle-shaped ZnO nanowires were around 100 nm
and 1 μm, respectively. The lengths of the nanowires were tens of microns, which could
be controlled by adjusting the growth time. In our experiment, the nanowire growth
was different from that of the conventional VLS process, where a catalyst (e.g., Au)
is necessary to promote a uni-axial growth, and the lattice match between the nanowires
and the substrate is critical for achieving vertically aligned nanowire arrays [22-24]. For the synthesis reported here, metal catalyst was not used and the growth followed
a vapor-phase transport deposition process. The zinc powder first evaporated slowly
after the furnace temperature was increased and formed a uniform thin seed layer of
ZnO on the FTO substrate. The zinc vapor pressure became higher as the temperature
was further increased, and a relatively high-zinc-concentration environment was formed
around the substrate location, and the nanowire growth was initiated. It is important
to point out that the source container played a critical role in the growth process.
Since the size of the source container was only several millimeters smaller than the
inner diameter of the tube, the container blocked the direct flow of the carrying
gas over the zinc power and prevented the evaporated zinc vapor from being transferred
too fast, which helped maintain the supersaturation level of the vapor that assisted
the nanowire growth. In fact, when the diameter of the source container was modified
to be smaller than the aforementioned dimension, the nanowire growth was significantly
affected, and for certain cases, there was no nanowire growth at all. The synthesized
nanowires showed very good mechanical attachment to the FTO substrates, and it was
very difficult to remove the nanowires by the typical ultrasonic method.

Figure 2.Vertically aligned ZnO nanowires on FTO substrates. (a) FESEM image (tilted at 15°) of a directly synthesized ZnO nanowire array on a FTO
substrate. The inset is a higher-magnification image. (b) XRD pattern of the ZnO nanowires grown on the FTO substrate. The inset shows the
transmission electron microscopy image of a single wire and the corresponding selected
area electron diffraction pattern.

ZnO nanowires with different lengths were synthesized in this research. The length
control of the nanowire arrays was realized by adjusting the growth time only while
keeping all the other growth parameters constant. Figure 3 exhibits the dependence of the nanowire array length on the growth time. The longest
nanowire array obtained for this experiment was 31 μm under a growth time of 25 min.
If the growth time was further increased, then more source material was needed in
order to produce longer wires. The inset in Figure 3 shows a cross-sectional FESEM image of an as-grown ZnO nanowire array with a length
of 31 μm on an FTO substrate.

Figure 3.ZnO nanowire array length dependence on the growth time. Inset is a cross-sectional FESEM image of a nanowire array with a length of 31 μm.

DSSCs based on directly synthesized and transferred ZnO nanowires

DSSCs were fabricated using both directly synthesized and transferred ZnO nanowires
in order to compare their performance. Figure 4a is the cross-sectional FESEM image of a ZnO nanowire film during the removing process,
where the major part of the film was peeled off but a small portion was still attached
to the silicon substrate. Figure 4b shows the bottom of the ZnO nanowire film after it was removed from the Si substrate,
which would be attached onto a FTO-coated substrate using silver paste. The advantages
of this transfer procedure were the following: the whole nanowire array could be transferred
at one time, the nanowires kept good vertical alignment [20], and the procedure did not cause a significant amount of broken wires inside the
nanowire array. A typical JV curve of a DSSC fabricated using the transferred ZnO nanowires is shown in Figure
4c. After subtracting the thickness of the PDMS layer, the effective nanowire length
for dye molecule loading of this device was 12 μm. The short-circuit current density
(Jsc) was 1.4 mA/cm2, and the open-circuit voltage (Voc) was about 0.32 V. These values were comparable to the previous reported results
of DSSCs fabricated using the same nanowire transfer procedure [20]. As a comparison, Figure 4c also shows the JV curve of a DSSC fabricated using directly synthesized ZnO nanowires that had the
same effective nanowire length of 12 μm for dye loading, which exhibited substantial
improvement on both Jsc and Voc. One indicator of solar cell performance is the series resistance (Rs) that can be estimated from a JV curve using [25]. To enhance the output efficiency of a solar cell, the cell's series resistance should
be minimized. The calculated Rs of the DSSC with transferred ZnO nanowires was 195 Ω·cm2, which was significantly larger than that of the cell fabricated using directly synthesized
nanowires (85 Ω·cm2). The major contributions to the series resistance of nanowire-DSSCs are the resistance
of the nanowire array, the contact resistance between the nanowires and the bottom
FTO electrode, the resistance between the nanowires and the electrolyte, the resistances
of the electrolyte and between the electrolyte and the counter FTO electrode, the
resistances of the two FTO electrodes, and the parasitic probe resistance. The major
difference in the device structures of the two types of DSSCs was the bottom contact
between the nanowire array and the FTO-coated substrate, and this increase in the
series resistance could be mainly attributed to the less-than-ideal bottom contact
in the DSSCs fabricated using transferred nanowires.

Figure 4.DSSCs based on directly synthesized and transferred ZnO nanowires. (a) Cross-sectional FESEM image of a ZnO nanowire film during the peeling-off process.
(b) FESEM image of the bottom of the ZnO nanowire film after removing from the silicon
substrate. (c) J-V plots of DSSCs fabricated using transferred and directly grown ZnO nanowires.

DSSCs based on directly synthesized ZnO nanowires with different lengths

DSSCs have been fabricated using the directly synthesized ZnO nanowires with different
lengths. Figure 5 shows the effect of the nanowire array length on cell performance parameters including
JscVoc, power conversion efficiency (η), and fill factor (FF). As Figure 5a,b shows, Jsc,Voc, and η of the DSSCs were improved when the nanowire array length became longer, which could
be explained by the increase in dye molecule loading due to the longer wires used.
The cell fabricated with the longest nanowires showed the best performance of Jsc of 5.1 mA/cm2Voc of 0.71 V, and η of 1.7%. This performance was among the best of ZnO nanowire-DSSCs [7,9,26,27].

Figure 5.Effect of the nanowire array length on cell performance parameters. (a) Plots of short-circuit current density and open-circuit voltage as functions of
nanowire array length. (b) The overall power conversation efficiency and fill factor as functions of nanowire
array length.

Effect of annealing on the performance of DSSCs

To investigate possible methods to enhance solar cell performance, the effect of annealing
on device performance was also studied. Directly synthesized ZnO nanowires on FTO
substrates with the same length of 31 μm were annealed at 550, 600, 650, 700, and
750 °C, respectively, under the same growth-forming gas environment, and DSSCs based
on the annealed nanowires were fabricated. To examine the annealing effect on dye
molecule loading, the absorption spectra of the N719 dye solutions after nanowire
sensitization were measured by a UV–vis spectrophotometer, and the results are shown
in Figure 6a. The temperatures in Figure 6a represent the different nanowire annealing temperatures, and the two peaks at 384
and 525 nm are the characteristic absorption peaks of the N719 dye. A higher absorption
intensity in Figure 6a corresponded to a larger amount of dye molecules left in the solution after nanowire
sensitization, thus indicating a smaller amount of dye loading on the nanowire surface.
As Figure 6a reveals, when the annealing temperature was increased, there was more dye molecule
loading on the ZnO nanowires. However, despite of the improved dye loading at higher
annealing temperatures, the fabricated solar cells actually showed decreased performance,
as Figure 6b shows. The inset in Figure 6b shows the calculated series resistances of the cells with annealed nanowires. As
it reveals, Rs increased significantly when the annealing temperature was increased. Figure 6c shows the XRD data of ZnO nanowires on FTO substrates annealed at different temperatures,
suggesting that the crystalline structure of the ZnO nanowires did not change significantly
after annealing. The increase in the series resistance was possibly due to the high-temperature
annealing damage to the FTO-coated substrates. We carried out a control study of bare
FTO-coated substrates annealed at same temperatures using the typical 4-point probe
measurement, and the FTO substrates' surface resistivities showed an increase at high
annealing temperatures. However, this increase alone could not justify the significant
change in the cell's series resistance. Another possibility could be an increased
formation of Zn2+ dye clusters on the nanowire surface after high-temperature annealing [28]. Such clusters could create additional barriers for electron transfer from the dye
molecules to the nanowires and cause degradation in device performance [28-30].

Figure 6.Annealing effects on the performance of DSSCs. (a) UV–vis absorption spectra of the N719 dye solutions after nanowire sensitization.
The temperature represents the nanowire annealing temperature. (b) J-V curves of the DSSCs fabricated using as-grown nanowires on FTO substrates annealed
at different temperatures. The inset shows the dependence of the series resistance
on the annealing temperature.(c) XRD data of ZnO nanowires on FTO substrates annealed at different temperatures.

Conclusions

In this research, we demonstrated a method to directly synthesize vertically aligned
long ZnO nanowires on FTO-coated glass substrates. The synthesis is based on a straightforward,
one-step CVD approach, which avoided the wet chemical processing in typical hydrothermal
growth and eliminated the nanowire transfer process for DSSC fabrication. DSSCs based
on these directly grown ZnO nanowires showed improved performance compared to those
fabricated using transferred nanowires. The performance of the DSSCs could be further
improved when longer nanowires were used. The relatively long nanowires provided an
alternative for hybrid nanowire/composite solar cells in efficiency enhancement [31,32]. The effect of the annealing temperature was also examined, and it was observed that
high annealing temperature caused a substantial increase in the cell's series resistance
and lowered the device performance. The reported direct synthesis approach could be
further improved and applied for the growth of other types of nanowires and could
benefit the fabrication of dye- or quantum dot-sensitized solar structures.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

LL performed the experiment and drafted the manuscript. JC and LL participated in
the experiment. WW supervised the work and finalized the manuscript. All authors read
and approved the final manuscript.

Acknowledgments

This work was supported by the US Department of Energy, Office of Basic Energy Sciences,
Division of Materials Sciences and Engineering under award DE-FG02-10ER46728 (materials
synthesis), and by the NASA EPSCoR under award NNX10AR90A (device characterization).